cwrapper trouble with bup on macOS

[Greg Troxel <gdt%lexort.com@localhost>]

For a long time I've had trouble building sysutils/bup on macOS with
cwrappers, and worked around it by USE_CWRAPPERS=no for that package.
Now, it's failing (differently) under oldwrappers.  Rather than address
that, it seems better to figure out what is wrong with cwrappers.

The symptom is bup's configure saying it can't run cpp as a pipeline:

  Looking for cpp (/opt/pkgsrc/sysutils/bup/work/.cwrapper/bin/cpp) (can't run cpp as a pipeline)


I then tried to run the wrappers by hand.  cc is fine.  cpp fails.
/usr/bin/cpp seems fine.

  $ CWRAPPERS_CONFIG_DIR=work/.cwrapper/config /opt/pkg/libexec/cwrappers/cc-wrapper 
  clang: error: no input files

  $ CWRAPPERS_CONFIG_DIR=work/.cwrapper/config /opt/pkg/libexec/cwrappers/cpp-wrapper
  exec failed

  $ echo foo | /usr/bin/cpp
  # 1 "<stdin>"
  # 1 "<built-in>" 1
  # 1 "<built-in>" 3
  # 383 "<built-in>" 3
  # 1 "<command line>" 1
  # 1 "<built-in>" 2
  # 1 "<stdin>" 2
  foo

This is an intel mac with macOS 13 and Command Line Tools.   Pretty much
everything else builds ok, that can be expected to build on mac.